Mention Dr. Judah Folkman's name to colleagues and patients and only the grandest descriptors come to their lips  words like "giant in his field," "visionary," "genius," and "ahead of his time." Credited with revolutionizing cancer treatment with the theory that preventing angiogenesis, or new blood-vessel growth, would starve tumors, the director of vascular biology at Children's Hospital and professor of pediatric surgery at Harvard Medical School died on Monday in Denver. He was 74.

Befitting his ever-active mind and lifestyle, Folkman died of an apparent heart attack in the airport en route to a scientific conference. "At 74, he was as vibrant as I remember him 20 years ago when I was in his lab," says Dr. William Li, director and co-founder of the Angiogenesis Foundation and a student of Folkman's in the 1980s. "He was bustling around, meeting colleagues, teaching students, and giving lectures."

Born in Cleveland, Ohio, in 1933, Folkman had a passion for medicine that began early, in visits to hospitals with his father, a rabbi. Keeping in mind his father's advice to be a "rabbi-like doctor," Folkman honed two often competing abilities, becoming both a razor-sharp researcher and a compassionate clinician. "He would take time to lecture students on how to interact with everyone," says Dr. Steven Brem, director of neurosurgery at Moffitt Cancer Center in Florida and a former student of Folkman's at Harvard Medical School. "I remember one of the things he said  'When you talk to a bereaved family who has just lost a loved one, talk to them as if the person is still alive, because the family has not absorbed the impact of his death.' That's the way I feel about him now; that he is still alive, and we will keep his legacy alive."

As a researcher, Folkman's energy and creativity were practically boundless. "He would work 21 hours a day," says Brem. "He was chairman of pediatric surgery at Children's Hospital, so he would do surgery and see patients during day, then at night he would have dinner from six to eight, then work in the lab from eight to two a.m." That dedication led Folkman to change the way cancer is treated today. His hunch, dating to his early days in the lab in the 1960s, that cancer tumors rely on the formation of new blood vessels for nourishment and growth, has since led to six FDA-approved anticancer drugs that weaken tumors by blocking their blood supply. "The idea was met with skepticism and ridicule back then, but he doggedly persisted in proving his ideas," says Li. "He lived long enough to see his idea transformed into new treatments for colon cancer, lung cancer, kidney cancer, liver cancer and multiple myeloma."

The road wasn't always easy, however. Folkman's first compound, which biotech companies rushed to test in people at the beginning of this decade, proved less effective in patients than in mice, giving skeptics yet another reason to doubt the approach. But that agent, dismissed by U.S. researchers, eventually won approval in 2005 for treating lung cancer in China, where it is extending the lives of non-small-cell lung cancer patients.

"He had an amazing mind," says Brem. Even before his pioneering work in cancer treatments, Folkman and a colleague, while in the Navy, perfected what they called a "leaky plastic," which later became the basis for the implantable, time-release contraceptive Norplant. Not only did Folkman's work on angiogenesis benefit cancer patients, but the same principles are now leading to novel treatments for reviving dying heart tissue, restoring circulation to tissues crippled by diabetes and improving vision in patients with macular degeneration. His theories may yet impact the treatment of other conditions, including obesity: "More recently, he had the idea that fat cells relied on blood vessels just like tumors, so you could control obesity by using blood vesselgrowth inhibitors," says Li, who was inspired by Folkman to co-found an organization that promotes research in the field.

"I know he was 74, but it is still a shock," says Brem. "He changed medicine. He is a true inspiration and an icon."